Trusses revolutionised construction by radically increasing the distances between upright supports for the roofs of buildings, and the decks of bridges. Trusses also create the amazing strength found in the infrastructure of cranes and high rise buildings. The top and bottom structural components of trusses are called chords, while the upright and angled members between chords are called the truss web. Trusses today are primarily made from wood or steel, and are designed by engineers to fit many construction spans and loads.
Pitched trusses are most commonly used for roofs, although they also are used to stabilise the frames of high rise buildings. There are more than 20 different styles of pitched trusses and engineers can design specific styles for particular uses. In the construction of homes and low rise commercial buildings, the most common styles of pitched trusses are Fink, Howe, Scissors, King Post and Queen Post. Each of these styles also has variations making it possible for the trusses to span longer distances.
Pitched truss styles used in commercial buildings will span 90 feet or more without intermediate supports. These styles include the Bowstring, Portal Frame, and Pitched Warren.
Parallel chord trusses support floors, flat roofs, and the decks of bridges. They are also widely used in high rise construction to support concrete floor decking and to add rigidity to the building frames. These trusses will carry very heavy loads.
One attractive feature of them, when used as part of the frame for buildings, is that they also create places to install utilities. The gaps in the truss web are often used to run electrical conduit, plumbing pipes, and heating, ventilation and air conditioning (HVAC) ducts.
There are substantial variations in the design of the truss web to account for a wide range of spans and loads. When used for flat roofs they are often ordered with a shallow slope to accommodate roof drainage. These trusses can be cantilevered at each end to create overhangs. Some versions of these trusses are trimmable to help account for slight variations in the structure as it is being built.
The truncated truss serves as an intermediate load-bearing truss that other trusses attach to. They are sometimes called girder trusses. These trusses make it possible to use trusses for hip roofs and to span long distances where roofs change direction. The truncated truss is set in a specified place on the building and then Jack trusses fan out from its sides to cover the shorter spans. Jack trusses typically resemble one-half of a pitched truss.
Truncated trusses are often designed for specific situations so they may take many forms. Some resemble pitched, or dual pitch trusses, but often with more robust chords and truss web members.
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